Summary The Sikorsky S-61L helicopter (N346AA), serial number61425, had just begun to lift a load of logs when the engine sound stopped. As the helicopter began to descend, the longline was dropped and the main rotor began to slow. The helicopter descended about 700feet with the rotor continuing to slow until the aircraft struck trees and the ground. Company personnel arrived at the accident site within 15minutes and extinguished a small fire in the wreckage. The helicopter was destroyed by the impact and both pilots sustained fatal injuries. Ce rapport est galement disponible en franais. Other Factual Information The helicopter was engaged in heli-logging operations at Wendle Creek, British Columbia, about 42nautical miles (nm) southeast of Prince George, British Columbia. The aircraft was owned and operated by Croman Corporation, a USA company operating in Canada, and was being flown by a crew of two Canadian pilots with the flying pilot in the left seat. The crew had begun flying at about 0610 Pacific daylight time1 on the day of the accident and was about one hour into the third cycle2 of the day at 1020 when the accident occurred. Visual meteorological conditions existed at the time with unlimited visibility and light winds. The helicopter was using a 200-foot longline and was picking up a load of logs from an area at 4200feet above sea level (asl), uphill from standing timber at the edge of the cut-block. After the engine sound stopped, white smoke was seen coming from the engine exhaust area for about three seconds, and the main rotor began slowing as the helicopter flew down the hillside, over the standing timber, toward the log-landing area. The rotor continued slowing and several seconds later, the helicopter struck trees then the ground at 3700feet asl in a logged-off area. Although the aircraft wreckage sustained some fire damage to the engines and engine area, it was determined that the fire was post-crash in origin. Neither engine fire extinguisher was discharged. Both pilots were appropriately certificated and trained for the type of flight being conducted. A review of records revealed that the crews' flight and duty times were in accordance with existing regulations, and nothing remarkable was found regarding the pilots' activities prior to the accident. The pilot-flying held a valid class-1 medical certificate which included a limitation requiring corrective lenses to be worn; he was wearing glasses on the day of the accident. The investigation could not establish with certainty whether the shoulder harnesses were worn by the pilots. Nevertheless, the violence of the accident suggests that the crash was not survivable. By design, the two fuel quantity gauge indicators retain their last position when electrical power is removed. On-site examination showed that the forward fuel gauge indicated 450pounds, and the aft fuel gauge indicated 425pounds. This would equate to about 104imperial gallons of fuel on board the helicopter at the time of the accident. The aft fuel tank bladder remained intact following the impact, and the forward fuel tank bladder sustained a small tear which was slowly leaking fuel. A total of 82imperial gallons of fuel were recovered from the wreckage. The flight manual states that .6imperial gallon (5pounds) is unusable fuel. Fuel samples taken from N346AA's fuel tank sumps in the morning, on the day of the accident, were sent for analysis following the accident. The samples were determined to be JetA, which is an approved fuel for this helicopter, and contained inconsequential amounts of water and particulate matter. The load of logs on the longline at the time of the accident was measured and found to weigh 8250pounds. Weight and balance calculations show that with 104imperial gallons of fuel and two pilots, the helicopter could have been loaded with up to 9950pounds on the longline without exceeding the 22000pound helicopter gross weight limit or the 10000pound hook weight limit. The helicopter was determined to have been within weight and balance limits at the time of the accident. The main- and tail-rotor blades demonstrated damage consistent with that found when there is little or no rotor rpm at impact. The helicopter is equipped with two General Electric CT58-140-1 engines. A post-accident teardown of the number-one engine (serial number 285-074L) and the number-two engine (serial number295-083) revealed that both had damage consistent with that found on engines that were not operating at impact. Although the engines were severely damaged by the impact and post-crash fire, no malfunction was found that would have caused the engines to shut down. The engines are not equipped with auto-relight systems. Each engine, however, is equipped with an overspeed governor that is designed to shut it down in the event of an overspeed. The main gear box (MGB), P/N S6135-20600-039, S/N A14-A24-64-580, had accumulated 1050hours time since overhaul (TSO). The recommended time between overhauls (TBO) on MGBs used in repetitive external lift operations (REL), such as heli-logging, is 1100hours. It is noteworthy that 11months prior to installation in the accident helicopter, this MGB had been removed from another S-61 helicopter (N9119S) and repaired due to a series of input freewheel unit (IFWU) slips3. A post-accident teardown/inspection of the MGB was conducted at an approved Sikorsky main gear box overhaul facility. The inspection revealed normal wear on all components except the IFWUs. The chip detectors were found to be clean, and the main oil filter did not contain any significant debris, but fine bronze particles were found in the oil. The damage pattern in the MGB revealed that the main rotor was not turning at impact. Inspection of the MGB found that, with the exception of the IFWUs, all damage was impact-related. An input freewheel unit (IFWU) is a mechanical device which functions as a one-way clutch, allowing a helicopter's engine to drive the rotor but preventing the rotor from driving the engine. Input freewheel units were originally installed on piston engine helicopters, their function being to allow a successful autorotation by preventing the main rotor from being slowed by engine drag. The right and left IFWUs had been overhauled by the operator on 12September2001, using new camshafts, roller retainers, rollers, supports (oilites), gear housings and gear housing bearings. At the time of the accident, the IFWUs had accumulated 532 hours, which is within the recommended TBO of 50050hours on IFWUs used for REL operations (this TBO had been recommended by Sikorsky as a direct result of the finding of increased IFWU wear in REL operations observed during the 1993TSB investigationA93P0051). Following the accident, the IFWUs were disassembled, inspected, and several component parts were tested. The right and left IFWUs exhibited similar wear and damage. The dimension of the IFWUs gear housings, camshafts, and roller retainers, even in the worn areas, was not abnormal for IFWU components that had been in service on a logging aircraft for 532hours. However, the condition of the worn areas of those components, and in particular the condition of the rollers and oilites, was not typical for IFWU components seen at overhaul. The rollers exhibited multiple flat spots, smeared metal, and bronze contamination. Hardness testing of the rollers, which are required to be through-hardened to RC 60-64, revealed that, with the exception of locally damaged areas on the surface, the rollers were consistently one to two points below the minimum required hardness. Metallographic examination revealed that the locally damaged areas on the surface did not etch normally; they appeared as white zones. A darker etching zone was present adjacent to and below the white zones. There was transition from the darker zone to the expected tempered martensite microstructures. Microhardness measurements verified that the hardness of the white zones was significantly higher than the specified hardness of the rollers, and that the hardness of the adjacent dark zone was significantly lower than the specified core hardness. The microstructure and microhardness observations are consistent with local overheating of the surface above the martensite temperature (approximately 1600degrees Fahrenheit) followed by rapid cooling by the IFWU lubricant, thus forming untempered martensite. The softer zone adjacent to the untempered martensite consisted of over-tempered martensite due to exposure to temperatures below the martensite forming temperature but above the tempering temperature used during manufacture of the rollers. The roller paths on the gear housings were pitted and showed signs of multiple disengagements and re-engagements (parallel dents left by the rollers). In Sikorsky's experience, this type of damage is not typical of over-running (freewheeling) in which the wear appears more uniform and without evidence of the parallel impressions and pitting. The camshaft flats exhibited roller impressions which averaged 0.00106inch but included impressions as deep as 0.0016inch. The camshaft flats also had wave-shaped areas of raised metal up to 0.0019inch high on the low (disengaged) side of the roller impressions. Any measurable wear on the camshaft flats is cause for rejection of the part at overhaul. Oilite bushings support the roller retainer. The retainer maintains alignment of the rollers relative to the camshaft flats and gear housing. The oilites in both IFWUs exhibited a variety of damage types such as bending, cracking, crushing, and wearing. A significant amount of very fine particles of oilite material was found on the rollers, roller retainer, camshaft, gear housing, and in the oil pooled in the MGB front cover and sump. Comparison of oilite manufacture data to data gathered during the oilite inspection indicates that the oilites were manufactured within their specified limits. During the post-crash inspection of N346AA's IFWUs, another heli-logging company operating a SikorskyS-61 aircraft reported an IFWU slip. That unit was inspected and showed damage similar to that found on N346AA's IFWUs. N346AA's ELT was not installed at the time of the accident; it had been removed and was being stored at the helicopter service/refuelling site, about one nm from the heli-logging area and accident site. Anecdotal information suggests that because normal heli-logging operations often trigger ELTs, it is common practice to remove ELTs for heli-logging operations and reinstall them for all other flights. The accident helicopter was being operated within sight of ground personnel who were able to locate the wreckage without the aid of an ELT. A review of the logbooks and maintenance records indicates that, with the exception of the ELT not being installed, the helicopter had been certificated, equipped, maintained, and operated in accordance with existing regulations and approved procedures.